The machining of nonmetallic materials such as wood, wood composites, and graphite composites is one of the most important and widely used processes in manufacturing. One of the most important machining operations is the use of routers to cut grooves in or finish the edges of a workpiece.
Routers, which are just one type of cutting tool, are manufactured in a number of different shapes and sizes. A double fluted router is shown in FIG. I. The mode of operation is essentially the same for all routers. The router mounts in the spindle of a rotary tool head and rotates about its longitudinal axis. The workpiece is moved into the router in a direction perpendicular to the longitudinal axis of the router. The cutting edge of the router meets the workpiece and cuts a groove as the workpiece is advanced. FIG. II illustrates this machining process.
The width of the groove cut by the router is equal to the diameter of the router. The depth of the groove is equal to the depth of cut, i.e. the distance from the top of the workpiece to the tip of the router. The cutting speed is the revolutions per minute (rpm) of the router. The feed rate is the rate of advancement of the workpiece measured in inches per minute (ipm).
A variety of materials have been used or suggested for routers, including tool steels, high speed steels, cast non-ferrous alloys, metal bonded carbides, and ceramics. The performance of these materials is measured by productivity, tool life, and cleanliness of the cut.
Productivity, which is the rate of removal of material from the workpiece, may be calculated based on the depth of cut, the width of cut, and feed rate. These three factors in turn are dependent on the cutting speed Tool life is measured in terms of the amount of material removed before failure. Failure occurs either suddenly by breakage or gradually by abrasion, corrosion, or dulling of the cutting edge. The cleanliness of cut is determined by the smoothness of the machined surface and by lack of defects such as burn marks.
Tool steel, high speed steel and cast non-ferrous alloys are restricted to relatively low cutting speeds because they all have critical temperature limitations. Carbide materials, such as tungsten carbide, can run at cutting speeds much higher than those of the steels. However, the carbides are often run at relatively low feed rates because they are relatively brittle and are susceptible to impact breakage Also, tungsten carbide suffers high-temperature corrosion as a result of the chemical interaction with the workpiece at the high temperatures generated when cutting materials such as wood or medium-density fiberboard. See Steward, "High Temperature Corrosion of Tungsten Carbide from Machining Medium-Density Fiber Wood", The Carbide and Tool Journal, January-February, 1986.
Ceramic materials, such as alumina, can operate at much higher speeds and temperatures than conventional steel and carbide materials. Ceramics, however, are even more brittle than carbides and tend to fracture catastrophically and unexpectedly when subjected to impact. Thus, ceramics can be operated only at quite low feed rates
References relating to the use of ceramics as cutting tools include U.S. Pat. Nos. 4,063,908 ("Ogawa"), 4,323,323 ("Lumby"), 4,343,909 ("Adams"), 4,366,254 ("Rich"), 4,526,875 ("Yamamoto"), 4,543,343 ("Iyori"), 4,554,201 ("Andreev"), and 3,514,828 ("Wale"); Japanese patent numbers 5,848,621 ("Yajima"), and 6,005,079 ("Mitsu Metal"); United Kingdom patent number 2,157,282 ("Santrade"); and pending U.S. patent application Ser. No. 830,773 ("Rhodes").
Of all these references, only Mitsu Metal, Santrade, and Rhodes suggest the use of whisker reinforced ceramics in cutting tools; and neither of these three disclose a method for cutting nonmetallic materials such as wood, wood composites, and graphite composite using a rotter comprised of alumina reinforced with silicon carbide whiskers. Rhodes, which is perhaps the closest reference, claims generally cutting tools comprised of alumina reinforced with silicon carbide whiskers It also discloses the use of such tools to cut metal.
Wale describes the use of ceramic or tungsten carbide as a cutting insert mounted by brazing or with adhesive on the flutes of high speed steel end mill cutters. The inserts are backed entirely by the highspeed steel center member.
It has been suggested that silicon carbide fiber reinforced ceramic can be used in various machine parts, including heat exchangers, molds, ,nozzles, turbines, valves and gears. See Japanese patents nos. 59-54680 and 59-102681. Such disclosures, however, are not particularly pertinent to the invention described herein, since such parts are not primarily subjected to impact stresses as part of their normal service environment. No mention is made of improved toughness or impact resistance nor are such properties of interest in the articles described.
It has also been disclosed that fracture toughness in ceramics can be improved by incorporation of silicon carbide whiskers into the ceramics Papers by Becher and Wei have described mechanisms for increase in toughness as related to whisker content and orientation. See Becher and Wei, "Toughening Behavior in SiC Whisker Reinforced Alumina", Comm. Am. Cer. Soc. (September, 1984); Wei, "Transformation Toughened and Whisker Reinforced Ceramics", Soc. Auto. Engrs., Proc. 21st Auto. Tech. Dev. Mtg., 201-205 (March, 1984); and Wei and Becher, "Development of SiC-Whisker-Reinforced Ceramics," Am. Ceram. Soc. Bull., p. 298-304 (February 1985). See also U.S. Pat. No. 4,543,345 ("Wei").
Neither the cited papers nor the Wei patent suggest the use of alumina reinforced with silicon carbide whiskers for cutting nonmetallic materials. Also, while these references disclose that alumina reinforced wit silicon carbide whiskers shows increased fracture toughness and flexural strength, they do not disclose thermal shock resistance, or the ability of the material to withstand the combination of severe shear, tensile, and compressive stresses and repeated impact loading experienced by a router.